The existence of the histamine H3 receptor has been known for several years and the receptor is of current interest for the development of new medicaments. Recently, the human histamine H3 receptor has been cloned. The histamine H3 receptor is a presynaptic autoreceptor located both in the central and the peripheral nervous system, the skin and in organs such as the lung, the intestine, probably the spleen and the gastrointestinal tract. Recent evidence suggests that the H3 receptor shows intrinsic, constitutive activity, in vitro as well as in vivo (ie it is active in the absence of an agonist. Compounds acting as inverse agonists can inhibit this activity. The histamine H3 receptor has been demonstrated to regulate the release of histamine and also of other neurotransmitters such as serotonin and acetylcholine. A histamine H3 receptor antagonist or inverse agonist would therefore be expected to increase the release of these neurotransmitters in the brain. A histamine H3 receptor agonist, on the contrary, leads to an inhibition of the biosynthesis of histamine and an inhibition of the release of histamine and also of other neurotransmitters such as serotonin and acetylcholine. These findings suggest that histamine H3 receptor agonists, inverse agonists and antagonists could be important mediators of neuronal activity. Accordingly, the histamine H3 receptor is an important target for new therapeutics.
Several publications disclose the preparation and use of histamine H3 agonists and antagonists. Most of these are imidazole derivatives. However, recently some imidazole-free ligands of the histamine H3 receptor have been described (see eg Linney et al., J. Med. Chem. 2000, 43, 2362-2370; U.S. Pat. No. 6,316,475, WO 01/66534, WO 01/74810.
In view of the art's interest in histamine H3 receptor agonists, inverse agonists and antagonists, novel compounds which interact with the histamine H3 receptor would be a highly desirable contribution to the art. The present invention provides such a contribution to the art being based on the finding that a novel class of substituted hexahydropyrrolo[1,2-a]pyrazines, octahydropyrido[1,2-a]pyrazines and decahydropyrazino[1,2-a]azepines has a high and specific affinity to and potency at the histamine H3 receptor. Compounds having a certain similarity to the compounds of the present invention have previously been prepared, and their biological properties have been investigated, cf. Decosta et al., J. Med. Chem. 36 (16) 2311-2320 (1993) and Bromidge et al., Bioorg. Med. Chem. Lett. 12 (10) 1357-1360 (2002). However, these references do not disclose that these compounds may have a histamine H3 receptor antagonistic or agonistic activity.
Due to their interaction with the histamine H3 receptor, the present compounds are useful in the treatment of a wide range of conditions and disorders in which an interaction with the histamine H3 receptor is beneficial. Thus, the compounds may find use e.g. in the treatment of diseases of the central nervous system, the peripheral nervous system, the cardiovascular system, the pulmonary system, the gastrointestinal system and the endocrinological system.